Methods of forming silicon germanium structures

ABSTRACT

Methods for forming structures that include a layer comprising silicon germanium are disclosed. Exemplary embodiments of the disclosure provide improved methods of forming a transition layer on the layer comprising silicon germanium that can mitigate any formation of an interface layer between the layer comprising silicon germanium and a subsequently formed layer comprising silicon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/875,489 filed Jul. 17, 2019 titled METHODS OF FORMING SILICON GERMANIUM STRUCTURES, the disclosures of which are hereby incorporated by reference in their entirety.

FIELD OF DISCLOSURE

The present disclosure generally relates to structures including silicon germanium layers and to methods of forming the structures. More particularly, the disclosure relates to methods of forming a transition and/or passivation layer on SiGe layers and to related structures.

BACKGROUND OF THE DISCLOSURE

Because of its relatively low bandgap and potentially relatively high carrier mobility, silicon germanium (SiGe) has been used in the manufacture of various electronic devices, such as photodetectors, thermoelectric devices, bipolar transistors, and field effect transistors, such as metal oxide field effect transistors (MOSFETS).

Recently, in efforts to form devices with increased device density, improved performance, and lower costs, three-dimension devices have been developed. For example, fin field effect transistors (FinFETS), in which a channel region is formed in the shape of a fin, allowing multiple points of contact to a channel region, and gate all around field effect transistors (Gate-All-Around FETs or GAA FETs), in which the gate surrounds all sides of a channel region, have been developed.

During the formation of GAA FETs, a stacked structure or superlattice of alternating layers comprising silicon (Si layers) and layers comprising SiGe (SiGe layers) can be formed by sequentially epitaxially growing the layers on a surface of a substrate. Portions of these layers can be etched to form (e.g., silicon) wires that can form channel regions of GAA FETS.

For many devices having Si layers epitaxially formed overlying SiGe layers, it is desirable to have a relatively small, abrupt transition between a SiGe layer and a Si layer epitaxially formed thereon. Unfortunately, Ge on the surface of the SiGe layer is energetically more favorable than Si on the surface. Consequently, if the surface of the SiGe layer is not passivated during the transition to the subsequent Si layer epitaxial growth, the Ge segregates, resulting in formation of an interfacial or interface layer. Then, as a first monoatomic layer of Si is grown, Ge segregates again to the top layer. This mechanism can be repeated during the growth of the Si layer, creating an interfacial layer of a few tens of angstroms.

SiGe layers can be grown using dichlorosilane (DCS) at temperatures between about 600° C. and 700° C. Such techniques result in relatively low growth rates of the SiGe layers (about 0.8 nm·min⁻¹) and can result in undesirably large interface layers with a subsequently grown Si layer.

Silane (SiH₄) at temperatures lower than 580° C. can be used to grow the Si layer overlying the SiGe layer. The growth of the silicon layer at such temperatures is relatively low growth rate (about 0.8 nm·min⁻¹) and the interface layer can still be undesirably thick.

Accordingly, improved methods of forming structures with SiGe layers, such as structures including Si layers epitaxially grown overlying the SiGe layers, are desired.

Any discussion, including discussion of problems and solutions, set forth in this section has been included in this disclosure solely for the purpose of providing a context for the present disclosure, and should not be taken as an admission that any or all of the discussion was known at the time the invention was made or otherwise constitutes prior art.

SUMMARY OF THE DISCLOSURE

Various embodiments of the present disclosure provide improved methods for forming structures including layers comprising SiGe (SiGe layers). While the ways in which the various drawbacks of the prior art are discussed in greater detail below, in general, exemplary methods include forming a transition layer using a plurality of silicon gases, which allows for formation of a relatively thin interface layer and can allow for higher temperatures and thus faster growth rates of layers comprising silicon (Si layers) that are epitaxially grown overlying the SiGe layers.

In accordance with at least one embodiment of the disclosure, a method of forming a structure is provided. An exemplary method includes providing a substrate comprising a layer comprising SiGe within a reaction chamber and forming a transition and/or passivation layer on the layer comprising SiGe by providing a gas comprising a mixture of: a first silicon-containing gas, a halogen-containing (e.g., chlorine-containing) gas, and a second silicon-containing gas within the reaction chamber, wherein the first silicon-containing gas is different from the second silicon-containing gas. The first silicon gas and the halogen/chlorine-containing gas can provide desired Ge-halogen or Ge—Cl bonds on a surface of the SiGe layer. The second silicon-containing gas can provide silicon to any vacant sites that may be on the surface—such as when a temperature of a substrate is above 600° C. A chemical formula of the first silicon-containing gas can include a halogen, such as chlorine. For example, the first silicon-containing gas can include one or more of chlorosilanes, such as dichlorosilane (DCS), chloropentamethyldisilane (CPMS), dichlorotetramethyldisilane (DTMS), pentachlorodisilane (PCDS), hexachlorodisilane (HCDS), and/or octachlorotrisilane (OCTS). The halogen/chlorine containing gas can be or include, for example, hydrogen chloride (HCl). The second silicon-containing gas can include one or more of silanes comprising silicon and hydrogen, for example, silanes represented by the formula Si_(n)H_(2n+2) (wherein n is from 1 to 4), such as silane (SiH4), disilane (Si₂H₆), trisilane (Si₃H₈) and tetrasilane (Si₄H₁₀). The gas comprising the mixture can include, for example, about 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of the first silicon-containing gas, about 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of the halogen-containing gas, and/or about 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of the second silicon-containing gas. A ratio of volumetric flowrates of the first silicon-containing gas to the second silicon-containing gas can range from about 0.001 to about 10000 or about 0.1 to about 10. Additionally or alternatively, a ratio of volumetric flowrates of halogen or chlorine-containing gas to the second silicon-containing gas can range from about 0.001 to about 10000. Exemplary methods can additionally include epitaxially forming the SiGe layer and/or epitaxially forming a Si layer over the SiGe layer. Two or more of the SiGe layer, the transition, and the Si layer can be formed within the same reaction chamber—e.g., without a vacuum break between steps of forming the SiGe layer, the transition, and/or the Si layer. In accordance with exemplary aspects of these one or more embodiments, the method includes steps of forming a SiGe layer and forming a Si overlying the SiGe layer, wherein the step of forming the SiGe layer is performed at a first temperature and the step of forming the Si layer is performed at a second temperature; the first temperature can be the same or different than the second temperature—e.g., the second temperature can be equal to or higher than the first temperature. For example, the first temperature can be between about 350° C. and about 800° C. and the second temperature can be between about 350° C. and about 800° C. or between about 500° C. and about 700° C. A gas used to form the SiGe layer can include 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of a silicon-containing reactant and/or about 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of a germanium-containing reactant. A gas used to form the Si layer can include 0.01 percent to about 100 percent or about 0.1 percent to about 50 percent or about 1 percent to about 15 percent of silicon-containing reactant.

In accordance with another embodiment of the disclosure, a method of forming a structure includes providing a substrate within a reaction chamber, forming a layer comprising SiGe on a surface of the substrate using a silicon-containing precursor, and forming a transition and/or passivation layer on the layer comprising SiGe by providing a gas comprising a mixture of a first silicon-containing gas, a halogen—e.g., chlorine-containing gas, and a second silicon-containing gas within the reaction chamber. The silicon-containing precursor and the first silicon-containing gas can be or include the same chemical formula. The first silicon-containing gas, the halogen—e.g., chlorine-containing gas, and the second silicon-containing gas can be or include any of the first silicon-containing gas, the halogen (e.g., chlorine)-containing gas, and/or the second silicon-containing gas noted above and elsewhere herein. Exemplary methods can additionally include a step of forming a layer comprising silicon. The steps of forming a layer comprising SiGe, forming a transition layer, and forming a layer comprising silicon can be repeated to form a multi-layered or lattice structure—e.g., a structure suitable for use in the formation of gate all around (GAA) structures and devices, such as GAA field effect transistors (GAA FETs).

In accordance with at least one additional embodiment of the disclosure, a structure is provided. The structure can be formed according to a method described herein. A structure can include a first layer or substrate comprising silicon, a layer comprising silicon germanium overlying the first layer or substrate comprising silicon, and a second layer comprising silicon overlying the layer comprising silicon germanium. A thickness of an interface layer between the second layer comprising silicon and the layer comprising silicon germanium can be less than 0.01 nm, less than 0.2 nm, less than 20 nm, less than 1000 nm, or between about 0.1 nm or 1 nm and about 5 nm.

These and other embodiments will become readily apparent to those skilled in the art from the following detailed description of certain embodiments having reference to the attached figures; the invention not being limited to any particular embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of exemplary embodiments of the present disclosure can be derived by referring to the detailed description and claims when considered in connection with the following illustrative figures.

FIG. 1 illustrates a method of forming a structure including a transition layer.

FIG. 2 illustrates a method of forming a structure in accordance with exemplary embodiments of the disclosure.

FIG. 3 illustrates a method of forming a structure in accordance with exemplary embodiments of the disclosure.

FIG. 4 illustrates a structure in accordance with at least one embodiment of the disclosure.

It will be appreciated that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of illustrated embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Although certain embodiments and examples are disclosed below, it will be understood by those in the art that the invention extends beyond the specifically disclosed embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention disclosed should not be limited by the particular disclosed embodiments described below.

The present disclosure generally relates to methods of forming structures that include a layer comprising silicon germanium (SiGe) and to structures formed using the methods. Exemplary methods described herein can be used to form structures that include a layer comprising SiGe and a layer comprising silicon (Si) epitaxially formed overlying the layer comprising SiGe, such that an interface layer between the layer comprising SiGe and the layer comprising Si can be relatively small, and such that a transition between the layer comprising SiGe and the layer comprising Si is abrupt. Further, exemplary methods can include forming the layer comprising silicon at relatively high temperatures, to allow for faster growth rates, while maintaining the relatively small interface layer.

In this disclosure, “gas” can refer to material that is a gas at room temperature and pressure, a vaporized solid and/or a vaporized liquid, and may be constituted by a single gas or a mixture of gases, depending on the context. A gas other than the process gas, i.e., a gas introduced without passing through a gas distribution assembly, other gas distribution device, or the like, may be used for, e.g., sealing the reaction space, which includes a seal gas, such as a rare gas. In some embodiments, the terms “precursor” and “reactant” can be used interchangeably and can each refer generally to a gas-phase compound that participates in the chemical reaction that produces, for example, a film or layer.

As used herein, the term “substrate” can refer to any underlying material or materials that may be used to form, or upon which, a device, a circuit, or a film may be formed. A substrate can include a bulk material, such as silicon (e.g., single-crystal silicon), other Group IV materials, such as germanium, or the like, and can include one or more layers overlying or underlying the bulk material. Further, the substrate can include various topologies, such as recesses, lines, and the like formed within or on at least a portion of a layer of the substrate.

As used herein, the term “epitaxial layer” can refer to a substantially single crystalline layer overlying a substantially single crystalline layer or substrate.

As used herein, the term “layer comprising silicon germanium” or “SiGe layer” can refer to a layer that includes silicon and germanium, such as an alloy of silicon and germanium. Layers comprising silicon germanium can be represented by the general formula of Si_(1-x)Ge_(x), where x is greater than zero and less than one. In some contexts, a layer comprising SiGe can include bulk material of SiGe. Layers comprising silicon germanium can include other elements, such as one or more of B, As, Ga, P, Sn, In, or the like. For example, layers comprising silicon germanium can include about zero or up to about 0.01 to about 95 atomic percent of one or more of such other elements.

As used herein, the term “layer comprising silicon” or “Si layer” can refer to a layer that includes silicon, such as a monocrystalline silicon layer. Layers comprising silicon can include other elements, such as dopants, including, for example, C, P, As, B, Sb or the like—e.g., up to about 0.01 to about 95 atomic percent of one or more of the other elements.

As used herein, the term “structure” can refer to a partially or completely fabricated device structure. By way of examples, a structure can include a substrate with a transition layer and/or passivation formed thereon. Structures can include additional layers, such as layers comprising SiGe and/or layers comprising Si.

As used herein, unless otherwise specified, percentages can refer to volumetric, mass, or weight percent, and can refer to an absolute or a relative percent.

Unless otherwise noted, methods described herein can be used to form a passivation and/or a transition layer. In some contexts, the passivation layer and the transition layer may be considered to be the same layer. Unless indicated otherwise, a method of forming of a transition layer can be used to form a transition layer and vice versa.

A transition or passivation layer formed using DCS and hydrogen chloride (HCl) can be used to stabilize germanium segregation and thereby mitigate a thickness of a passivation layer between a SiGe layer and a Si layer. The bond between Si and CI is more favorable than the bond between Ge and Cl. Accordingly, use of DCS and HCl can help keep Si at the top layer of the SiGe layer. However, theoretically, above 600° C., the CI bonds to Si and might form SiCl₂ and desorb from the surface. This would lead to a vacancy into which Ge can move.

FIG. 1 illustrates a process in which DCS and germane (GeH₄) are used to epitaxially grow a SiGe layer, DCS and HCl are used to form the transition layer, and SiH₄ is used to grow a silicon layer on the SiGe layer. Use of such a technique can reduce a thickness of the interface layers between SiGe and Si layers; however, the thickness of the interface layer may still be undesirably large. And, the growth rate of the Si layer may be undesirably low.

FIG. 2 illustrates a method 200 in accordance with at least one embodiment of the disclosure. Method 200 includes the steps of providing a substrate comprising a layer comprising SiGe (step 202) and forming a transition layer (step 204). Method 200 can also include the steps of forming a layer compromising SiGe (step 206) and/or a step of forming a layer comprising Si (step 208), and optionally a repeat loop (step 210).

Step 202 can include providing a substrate as described herein within a reaction chamber. By way of examples, the substrate can include bulk silicon, a layer comprising Si, and/or a layer comprising SiGe. If the substrate includes a layer comprising SiGe on a surface of the substrate, method 200 can proceed to step 204. If the substrate does not include a layer comprising SiGe, then method 200 can proceed to step 206 prior to proceeding to step 204.

Step 204 includes forming a transition (and/or passivation) layer on the layer comprising SiGe. The transition layer can be formed by providing a gas comprising a mixture of two or more silicon-containing gases and a halogen (e.g., chlorine)-containing gas to the reaction chamber. For example, the mixture can include a first silicon-containing gas, a halogen—e.g., chlorine-containing gas, and a second silicon-containing gas. A thickness of the transition layer can range from about 0.01 to about 20 nm.

A temperature within the reaction chamber during step 204 can range from about 350° C. to about 800° C. or about 500° C. to about 700° C. During step 204, a temperature can ramp or change—e.g., from a temperature used during step 206 to a temperature used during step 208. Alternatively, the temperature can be the same as the temperature of step 206 and/or step 208 or different from a temperature used in either step 206 or step 208. A pressure within the reaction chamber during step 204 can range from about 1 to about 760 torr or about 25 to about 40 Torr.

A chemical formula of the first silicon-containing gas can comprise chlorine. By way of examples, the first silicon-containing gas can be or include dichlorosilane (DCS), chloropentamethyldisilane (CPMS), dichlorotetramethyldisilane (DTMS), hexachlorodisilane (HCDS), pentachlorodisilane (PCDS), and/or octachlorotrisilane (OCTS).

A chemical formula of the halogen-containing gas can comprise a halogen (e.g., chlorine) and hydrogen. By way of examples, the halogen-containing gas can be or include hydrogen chloride (HCl).

A chemical formula of the second silicon-containing gas can comprise hydrogen. By way of examples, the second silicon-containing gas can be or include a silane, such as silane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈) and/or tetrasilane (Si₄H₁₀).

Flowrates of the first silicon-containing gas, the halogen/chlorine-containing gas, and the second silicon-containing gas can vary according to a number of factors, including a size of the substrate, a pressure within the reaction chamber, and a size of a reaction chamber. In accordance with examples of the disclosure, a flowrate of the first silicon-containing gas into the reaction chamber can be about 0.01 to about 10000 sccm or about 75 to about 250 sccm; a flowrate of the halogen-containing gas into the reaction chamber can be about 0.01 to about 10000 or about 200 to about 300 sccm; and a flowrate of the second silicon-containing gas into the reaction chamber can be about 0.01 to about 10000 or about 50 to about 75 sccm. A ratio of volumetric flowrates of the first silicon-containing gas to the second silicon-containing gas ranges from about 0.01 to about 100. Additionally or alternatively, a ratio of volumetric flowrates of halogen/chlorine-containing gas to the second silicon-containing gas ranges from about 0.01 to about 100 or about 0.1 to about 10. In accordance with further examples, the gas comprising the mixture can include about 0.01 to about 100 of the first silicon-containing gas, about 0.01 to about 100 percent of the halogen-containing gas, about 0.01 to about 100 percent of the second silicon-containing gas, and/or other percentages and compositions as set forth herein.

Referring again to FIG. 2, step 206 includes forming a layer comprising SiGe—e.g., if the substrate provided during step 202 did not include a layer comprising SiGe on a surface of the substrate. Step 206 can include forming an epitaxial layer comprising SiGe—e.g., over bulk material comprising silicon or over a layer comprising silicon.

The (e.g., epitaxial) layer comprising SiGe can be formed by coflowing or alternating flow of two or more reactants into the reaction chamber. For example, a first reactant comprising silicon and a second reactant comprising germanium can be flowed into the reaction chamber. Exemplary first reactants comprising silicon include DCS, chloropentamethyldisilane (CPMS), dichlorotetramethyldisilane (DTMS), hexachlorodisilane (HCDS), pentachlorodisilane (PCDS), and/or octachlorotrisilane (OCTS). Exemplary second reactants comprising germanium include germane (GeH₄) and digermane (Ge₂H₆). A thickness of the layer comprising SiGe formed during step 206 can range from about 0.2 to about 1000 nm or about 2 to about 20 nm.

A temperature within the reaction chamber during step 206 can range from about 350° C. to about 800° C. or about 500° C. to about 700° C. A pressure within the reaction chamber during step 206 can range from about 1 Torr to about 760 Torr.

Step 208 includes forming a layer comprising Si—e.g., overlying the layer comprising SiGe. Step 208 can include forming an epitaxial layer comprising Si—e.g., over an epitaxial layer comprising SiGe formed during step 206. A thickness of the layer comprising SiGe formed during step 208 can range from about 0.2 to about 1000 nm.

A temperature within the reaction chamber during step 208 can range from about 350° C. to about 800° C. or about 500° C. to about 700° C. A pressure within the reaction chamber during step 208 can range from about 1 Torr to about 760 Torr.

In accordance with further examples of the disclosure, during any of steps 204-208, a gas or gas mixture may further include one or more carrier gases, which can include one or more inert gases, to facilitate the transport of reactants to the substrate surface. For example, hydrogen (H₂) and/or nitrogen (N₂) gases may be utilized as carrier gases. To mitigate unwanted reactions, steps 204-208, or any combination thereof, can be performed within the same reaction chamber without a vacuum break.

FIG. 3 illustrates various steps of method 200 in accordance with illustrative examples of the disclosure. For the examples illustrated in FIG. 3, the step of forming the layer comprising SiGe (e.g., an epitaxial layer comprising SiGe) is performed at a first temperature and the step of forming the layer comprising Si (e.g., an epitaxial layer) is performed at a second temperature, wherein the first temperature can be the same or different than the second temperature; for example, the second temperature can be greater than or equal to the first temperature. In the particular illustrated example, the second temperature is higher than the first temperature. Using a second temperature that is higher than the first temperature allows for higher growth rates of the layer comprising silicon. Growth of an interface layer that might otherwise occur is mitigated by the use of the second silicon-containing gas, which is thought to contribute silicon atoms to a surface of the substrate during formation of the transition layer to thereby mitigate germanium segregation to a surface of the layer comprising SiGe during the formation of the transition layer and/or during formation of the layer comprising silicon.

FIG. 4 illustrates a structure 400 in accordance with examples of the disclosure. Structure 400 includes a (e.g., silicon) substrate 402, a layer comprising SiGe 404 overlying substrate 402, and a layer comprising Si 406 overlying layer comprising SiGe 404.

Substrate 402 can be or include a bulk silicon substrate. Alternatively, a substrate having a monocrystalline layer comprising silicon could form part of structure 400.

Layer comprising SiGe 404 can be formed according to step 206 described above. Layer comprising SiGe 404 can be an epitaxial layer formed overlying substrate 402.

Layer comprising Si 406 can be formed according to step 208 described above. Layer comprising Si 406 can be an epitaxial layer formed overlying layer comprising SiGe 404.

A transition between layer comprising SiGe 404 and layer comprising Si 406 is abrupt—e.g., having a thickness as noted above.

The example embodiments of the disclosure described above do not limit the scope of the invention, since these embodiments are merely examples of the embodiments of the disclosure; the invention is defined by the appended claims and their legal equivalents. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the disclosure, in addition to those shown and described herein, such as alternative useful combination of the elements described, may become apparent to those skilled in the art from the description. Such modifications and embodiments are also intended to fall within the scope of the appended claims. 

What is claimed is:
 1. A method of forming a structure, the method comprising the steps of: providing a substrate comprising a layer comprising SiGe within a reaction chamber; and forming a transition layer on the layer comprising SiGe by providing a gas comprising a mixture of: a first silicon-containing gas; a halogen-containing gas; and a second silicon-containing gas within the reaction chamber, wherein the first silicon-containing gas is different than the second silicon-containing gas.
 2. The method of claim 1, wherein a chemical formula of the first silicon-containing gas comprises chlorine.
 3. The method of claim 1, wherein the first silicon-containing gas comprises one or more of dichlorosilane (DCS), chloropentamethyldisilane (CPMS), dichlorotetramethyldisilane (DTMS), hexachlorodisilane (HCDS), pentachlorodisilane (PCDS), and octachlorotrisilane (OCTS).
 4. The method of claim 1, wherein the halogen-containing gas comprises hydrogen chloride (HCl).
 5. The method of claim 1, wherein the second silicon-containing gas comprises one or more of silane (SiH₄), disilane (Si₂H₆), trisilane (Si₃H₈) and tetrasilane (Si₄H₁₀).
 6. The method of claim 1, wherein the gas comprising the mixture comprises about 0.01 to about 100 percent of the first silicon-containing gas, about 0.01 to about 100 percent of the halogen-containing gas, and about 0.01 to about 100 percent of the second silicon-containing gas.
 7. The method of claim 1, wherein a ratio of volumetric flowrates of the first silicon-containing gas to the second silicon-containing gas ranges from about 0.001 to about
 10000. 8. The method of claim 1, wherein a ratio of volumetric flowrates of halogen-containing gas to the second silicon-containing gas ranges from about 0.001 to about
 10000. 9. The method of claim 1, wherein the layer comprising SiGe is an epitaxial layer, and wherein the method further comprises a step of forming the epitaxial layer.
 10. The method of claim 9, wherein the step of forming the epitaxial layer and the step of forming the transition layer are performed within the same reaction chamber without a vacuum break.
 11. The method of claim 9, further comprising a step of forming a layer comprising silicon overlying the layer comprising SiGe, wherein the step of forming the epitaxial layer is performed at a first temperature and the step of forming the layer comprising silicon is performed at a second temperature, and wherein the first temperature is different than the second temperature.
 12. The method of claim 9, further comprising a step of forming a layer comprising silicon overlying the layer comprising SiGe, wherein the step of forming the epitaxial layer is performed at a first temperature and the step of forming the layer comprising silicon is performed at a second temperature, and wherein the second temperature is greater than or equal to the first temperature.
 13. The method of claim 9, wherein the step of forming a transition layer is performed without temperature change and/or at a third temperature.
 14. A method of forming a structure, the method comprising the steps of: providing a substrate within a reaction chamber; forming a layer comprising SiGe on a surface of the substrate using a silicon-containing precursor; and forming a transition layer on the layer comprising SiGe by providing a gas comprising a mixture of: a first silicon-containing gas; a halogen-containing gas; and a second silicon-containing gas within the reaction chamber.
 15. The method of claim 14, wherein the silicon-containing precursor and the first silicon-containing gas comprise the same chemical formula.
 16. The method of claim 14, wherein the steps of forming a layer comprising SiGe and forming a transition layer are repeated.
 17. The method of claim 14, wherein the second silicon-containing gas comprises a silane.
 18. A structure formed according to a method of claim
 1. 19. The structure of claim 18, wherein the structure comprises a first layer comprising silicon, a layer comprising silicon germanium overlying the first layer comprising silicon, and a second layer comprising silicon overlying the layer comprising silicon germanium.
 20. The structure of claim 19, wherein a thickness of an interface layer between the second layer comprising silicon and the layer comprising silicon germanium is about 0.01 nm to about 20 nm. 